1. Field of the Invention
This invention relates to cameras and particularly to camera flash photography systems with automatic exposure controls.
2. Description of the Prior Art
Known automatic exposure control systems for flash cameras include three types. In the first type, a light sensitive element in a flash receives light reflected from an object to be photographed and the flash's light is adjusted by stopping the flash emission when the amount of light reaches a predetermined value.
A second type involves a TTL light adjusting system wherein the light sensitive element is within a camera and receives light emitted from a flash and reflected from an object to be photographed through the camera's lens aperture. The flash illumination is adjusted by stopping the flash emission when the quantity of the light received reaches a prescribed value.
A third system adjusts a flash's light emission by computing a guide number on the basis of an aperture setting value and the distance information obtained from the distance ring of a lens or its position, and by controlling the flash emission according to the computed guide number.
However, all of these prior art systems have shortcomings which render them incapable of insuring a correct exposure.
In the first of these systems, namely, the external light adjusting system, the light sensitive element is independent of the camera's main optical system. Thus, the picture is not taken with the same light that reaches the exposure adjusting light sensitive element. Therefore, the light quantity for the main photographic region cannot accurately be detected. Also, the quantity of light received by the light sensitive element varies with the reflection factor of the the object to be photographed. It is thus impossible to make a correct exposure.
In the second, or TTL light adjusting system, the quantity of light received also varies with the reflection factor of the object being photographed. Therefore, a correct exposure is scarely possible.
In the third system, the one which adjusts the flash's emission on the basis of the distance, the light emission is determined by the aperture value and the object distance. Hence, adjustment errors in the object distance and the aperture value are inevitably reflected in the precision of the light adjustment.
The above shortcoming of the system which adjusts flash emission on the basis of distance (hereinafter called the distance light adjusting system) is particularly significant when used with a wide angle lens. The probability of obtaining a correct exposure drops greatly when a wide angle lens used. The reason for this is as follows:
To simplify the explanation, we consider an example where a single lens is in use. Assuming the distance from the surface of a film to an object to be photographed is R, the focal length of the lens is f and a distance from the film surface to lens position is X as shown in FIG. 1 of the accompanying drawings, the following relation obtains: EQU 1/X+1/(R-X)=1/f
Assuming that the lens is shifted relative to the film surface from a position focusing on infinity .infin. to a position focusing on another object at a distance R over a distance .DELTA.X, the lens shift .DELTA.X can be expressed as: EQU .DELTA.X=f2/(R-2f)
Therefore, the lens shift .DELTA.X required for focus adjustment to a given object distance is smaller when the focal length f of the lens is shorter. Conversely, when the shift .DELTA.X is assumed to be unvarying, a shorter focal length lens produces a wider range over which an object is in focus within the scope of the lens shift than a lens of longer focal length. This means that even a very small shift of a shorter focal length lens changes the focusing distance substantially. In other words, with a wide angle lens, the lens position precision has a greater influence on the focusing distance than a lens whose angle is not as wide. Where the setting of a photo-taking lens is detected by a position detecting device, and an object distance setting value signal is obtained from the position detection, an error in the distance value signal is determined by the precision of the detecting device. In other words, if detecting device of the same detecting precision is used for detecting the setting of a telephoto lens and that of a wide angle lens, the signal representing a detected distance will have a greater error with the wide angle lens.
Let us now assume that the precision of the position detecting device for detecting the setting of the photo-taking lens is .+-.0.1 mm. In this condition, when the lens is shifted .DELTA.X to focus it on an object located at a distance of 4 m, and then the shifted position of the lens is detected by the position detecting device, the above-stated degree of precision of .+-.0.1 mm causes the detecting device to detect the lens shift as .DELTA.X .+-.0.1 mm. Accordingly, the detecting device produces a signal representative of a preset distance value corresponding to the lens shift of .DELTA.X .+-.0.1 mm. However, the setting of the lens is 4 m when the lens shift is .DELTA.X as mentioned. With the lens position shift thus deviating .+-.0.1 mm from .DELTA.X, the distance setting range of a 35 mm f=35 mm, is from 3 m to 5.9 m as shown in FIG. 2 of the accompanying drawing. Thus, the detection error of .+-.0.1 mm causes the setting distance value signal to represent 3 m-5.9 m despite of the fact that the lens has been set for a distance of 4 m by shifting it .DELTA.X. With the lens whose f=35 mm, therefore, the setting distance of 4 m results in an error between -1 m and +1.9 m.
With a 28 mm lens, i.e., f=28 mm, which has a wider angle than the 35 mm lens, the .+-.0.1 mm error for the lens shift .DELTA.X affects the distance range to as great a degree as the ratio of the variation of the lens setting distance value to the variation of the lens shift and is greater in a wide angle lens than in a telephoto lens as mentioned in the foregoing. In this instance, as shown in FIG. 2, the setting distance range becomes 2.7 m-8 m with the 28 mm lens shifted .DELTA.X .+-.0.1 mm. The above-stated detection error of .+-.0.1 mm then causes the distance signal to represent 2.7 m-8 m while the distance is set for 4 m by shifting the lens .DELTA.X. Therefore, with a 28 mm lens the distance setting of 4 m results in an error between -1.3 m and +4 m. Where a lens setting is detected by a distance setting detecting device to obtain a distance setting value signal in this manner, the detecting precision of the detecting device affects the accuracy of the distance setting value signal. The signal error increases as the angle of the wide angle lens increases.
In the aforementioned distance light adjusting method, a guide member is obtained by computing the product of the aperture value and value of the distance to the object and the quantity of light to be emitted by the flash is made to correspond to the guide member to ensure a correct exposure. Therefore, when a distance setting value is obtained by the position detecting device and the guide member is obtained by computation on the basis of the distance value thus obtained, the detecting error affects the accuracy of an exposure substantially. The error in the detected distance value signal relative to the distance setting value of the lens becomes very high particularly when a wide angle lens is used.
FIG. 3 shows the degrees of deviation from a correct exposure when the quantity of a flash's light is controlled by obtaining a lens setting distance value signal using the aforementioned position detecting means. When the object to be photographed is located at a distance of 4 m, for example, the range of possible deviation from the correct exposure is from an under-exposure by about 1.2 step to an over-exposure by about 2 steps with a 28 mm lens and is from an under-exposure by about 0.8 step to an over-exposure by about 1.1 step with a 35 mm lens as shown in FIG. 3.
Therefore, light adjustment based on distance is not suitable for flash photography with a wide angle lens because of the extremely low probability of a correct exposure.
These disadvantages are applicable not only to light adjusting systems which respond to a distance in order to control the total flash emission, but to all systems which limit the flash emission on the basis of a distance. For example, they apply to exposure control for a flash by using a distance value signal and a guide number to set an aperture.
Another disadvantage of light adjustment on the basis of distance arises from the fact that the flash output is determined by the aperture and the distance to an object. Hence, when the light coming through the objective lens is altered by an ND filter or the like, the exposure will be diminished by the light reduction through the objective lens.
Furthermore, the use of intermediate adaptor that alters the focal length, such as a teleconverter also makes it impossible to obtain a correct exposure. In other words, a device such as an extension ring for close-up photography will multiply the actual aperture by (M+1) and the aperture will make the picture darker than the aperture value indicated by the graduations on the lens due to the increase in magnification and multiplication by M. This results in a difference between the actual aperture value and the set aperture value or value indicated by the lens graduations. Therefore, the so-called light adjusting arrangement which controls the flash emission on the basis of an aperture setting and a set distance hardly insures a correct exposure.